The present invention relates to an optical pickup device for effecting the reproduction of an optical recording medium. More particularly, the present invention concerns an optical system for separating emergent light from a light source and return light from the optical recording medium.
As an optical pickup device for effecting the reproduction of an optical recording medium such as a compact disk (CD), an optical pickup device of a polarizing system is known which is arranged to be capable of separating emergent light and return light by causing the emergent light from the laser light source and the return light from the optical recording medium to pass through a quarter-wave phase difference plate (quarter-wave plate). For example, as shown in FIGS. 10 and 11, a device in which a polarizing beam splitter 11 (polarizing and separating element), a phase difference plate 12, and an objective lens 16 are disposed at positions midway in the optical path from a laser light source to a photodetector is arranged such that the light emitted from a laser light source 13 constituted by a laser diode passes through the polarizing beam splitter 11 and the phase difference plate 12, and is then applied to the recording surface of an optical recording medium 5 as a spot of light, while the return light from the optical recording medium 5 passes through the phase difference plate 12 and the polarizing beam splitter 11 again. The return light from the optical recording medium 5, when passing through the phase difference plate 12, is converted to laser light whose polarization direction differs 90xc2x0 from the polarization direction of the emergent light from the laser light source 13, and is guided to a photodetector 14 disposed in a direction different from that of the laser light source 13 by the polarizing beam splitter 11.
In addition, there are cases in which, as shown in FIGS. 1 and 2, a diffractive element 21 (hologram element) serving as a polarizing and separating element is disposed between the laser light source 13 and the phase difference plate 12, and the return light from the optical recording medium 5 is guided to the photodetector 14 by this diffractive element 21.
In either one of these optical pickup devices 1A and 1B of the polarizing system, as a development of the optical system and the state of polarization of the light are schematically shown in FIGS. 12A and 12B, the linearly polarized light emitted from the laser light source 13 is converted to circularly polarized light by the phase difference plate 12, and the return light (circularly polarized light) from the optical recording medium 5 is converted to linearly polarized light whose polarization direction differs 90xc2x0 from the polarization direction of the linearly polarized light emitted from the laser light source 13 by the phase difference plate 12, and is guided to the photodetector 14 disposed in a direction different from that of the laser light source 13.
Accordingly, with the optical pickup devices 1A and 1B of this type, there is an advantage in that the emergent light from the laser light source 13 can be applied effectively to the optical recording medium 5, and the return light from the optical recording medium 5 can be guided to the photodetector 14 with high efficiency.
However, in the above-described optical pickup device, the effective quantity of light (represented as EQL in the figure) becomes 100% when the amount of birefringence xcex4 of the optical recording medium 5 is 0. In a case where the optical recording medium 5 itself has birefringence, a change occurs in the polarized state of the light due to this birefringence as well, so that there is the problem that the effective quantity of light drops below 100%.
For example, as shown in FIG. 12C, if the optical recording medium 5 itself has an amount of birefringence xcex4 corresponding to the quarter wavelength in the back-and-forth movement of the light, the light already becomes linearly polarized light when it is reflected by the optical recording medium 5. Consequently, when the return light reflected by the optical recording medium 5 passes through the phase difference plate 12 again, the linearly polarized light becomes circularly polarized light, and the effective quantity of light drops to 50%. Further, as shown in FIG. 12D, if the optical recording medium 5 has an amount of birefringence xcex4 corresponding to the xcex/2 (hereinafter, wavelength is represented by xcex) in the back-and-forth movement of the light, the return light, when passing through the phase difference plate 12, is converted at this point of time to the linearly polarized light whose polarization direction is the same as that of the linearly polarized light emitted from the laser light source 13. Consequently, the return light and the emergent light from the laser light source 13 cannot be separated from each other, so that the effective quantity of light reaching the photodetector 14 becomes 0%.
The relationship between the amount of birefringence xcex4 of such an optical recording medium 5 and the detected quantity of light can be expressed as the relationship such as the one indicated by the dotted line L0 in FIG. 5. Namely, if the quantity of light detected by the photodetector 14 when the amount of birefringence xcex4 of the optical recording medium 5 is 0 is set as 1, the intensity of the signal detected by the photodetector 14 continues to drop when the amount of birefringence xcex4 of the optical recording medium 5 shifts from 0 to xcex/2. When the amount of birefringence xcex4 of the optical recording medium 5 reaches xcex/2, the signal intensity becomes 0.
Generally, the substrate of the optical recording medium 5 is fabricated by injection molding, and since the resin flows radially outward from the central side of the optical recording medium 5, the optical recording medium 5 is likely to have birefringence whereby the refractive index differs between the radial direction and the circumferential direction. When the signal intensity was measured from the central side to the radially outward side of the optical recording medium 5 to confirm its actual state, there was an optical recording medium exhibiting the characteristic shown in FIG. 13 as an example which exhibited extreme birefringence. In the characteristic shown in FIG. 13, the signal intensity was initially at an extremely low level on the central side of the disk, and exhibited a minimum value at a position offset slightly toward the radially outward side therefrom, and the signal intensity became gradually higher in a region extending from that position toward the radially outermost side. If consideration is given on the basis of this result, it can be seen that, in the disk having the characteristic shown in FIG. 13, there is a region P where the amount of birefringence xcex4 is xcex/2 on the slightly radially outward side from the center, and that the amount of birefringence dxcex4 becomes gradually smaller on the further radially outward side. Although this example is an extreme one, it can be estimated that disks which are manufactured by the same manufacturing method show a similar tendency, and it is conceivable that such disks generally have certain quantities of birefringence in the entire radial direction.
In contrast, it is conceivable to use an optical pickup device which is arranged such that the optical recording medium 5 itself and the phase difference plate 12 function as a single phase difference plate by orienting the direction of the anisotropic axis (hereinafter simply referred to as the azimuth) of the phase difference plate 12 in the direction of the birefringence of the optical recording medium 5, and in which, instead of the quarter-wave phase difference plate, a phase difference plate having a phase difference with the amount of phase difference offset from the quarter wavelength by a portion corresponding to the amount of birefringence of the optical recording medium 5 itself is used as the phase difference plate 12.
If such an arrangement is adopted, since the phase difference plate 12 and the optical recording medium 5 itself together function as a quarter-wave phase difference plate, even if the optical recording medium 5 has birefringence, the light emitted from the laser light source 13, after the transmittance of the return light through the phase difference plate 12, is converted to linearly polarized light whose polarization direction differs 90xc2x0 from that of the emergent light from the laser light source 13. Accordingly, even if the optical recording medium 5 has birefringence, it is possible to configure an optical pickup device having a high effective quantity of light.
In a case where such an arrangement is adopted, it is the general practice to set the polarization direction of the laser light emitted from the laser light source 13 and the axial direction of the phase difference plate 12 at an angle of 45xc2x0. Such a setting is generally effected by rotating the laser light source 13 about the optical axis of the emergent light to adjust its angular position. However, with such an adjustment method, the laser light source 13 can be rotated singly about the optical axis of the emergent light in the case where the laser light source 13 and the photodetector 14 are formed separately, as shown in FIGS. 10 and 11. However, such an adjustment method cannot be adopted in the case where the laser light source 13 is formed integrally with the photodetector 14 as a light source unit 20 as in the case of the optical pickup device 1A shown in FIGS. 1 to 3. Hence, there is a problem in that a large restriction is imposed on the layout of the optical system.
In view of the above-described problems, an object of the invention is to provide an optical pickup device which is capable of effecting stable optical detection without imposing a large restriction on the layout of the optical system even if the optical recording medium itself has birefringence.
According to the invention, the phase difference and azimuth of the phase difference plate are set so that a peak of the signal intensity appears in the photodetector while the amount of birefringence of the optical recording medium changes from 0 to xcex/4 without placing a precondition on aligning the azimuth of the phase difference plate with the direction of birefringence of the optical recording medium.
In addition, the optical recording medium is irradiated through the phase difference plate whose azimuth is oriented in a range of 50xc2x0 to 60xc2x0 with respect to laser light emitted from the laser light source.
Further, the polarization direction of emergent light from the laser light source is arranged so as to be oriented at 45xc2x0 with respect to the radial direction of the optical recording medium, and the azimuth of the phase difference plate is offset in a range of about 5xc2x0 to 15xc2x0 with respect to the 45xc2x0.
Accordingly, in the invention, even if the azimuth of the phase difference plate is not oriented in the direction of birefringence of the optical recording medium, the return light from the recording medium can be detected with high intensity in the range in which the amount of birefringence of the optical recording medium is between 0 to xcex/4. Therefore, it is possible to arrange an optical pickup device with a high effective quantity of light. In addition, the light source can be arranged in an optimal state in view of the design feature and the like irrespective of in whichever direction the birefringence of the optical recording medium is actually oriented. Accordingly, even in the optical pickup device in which the light source and the photodetector are integrally formed, stable signal detection can be effected by absorbing the birefringence of the optical recording medium.